Wetted surface area of hull / hull construction mass weight

[gertha]

If my boat has a heavier construction , as in the materials used are of a greater mass, than another boat of the same displacement, presumably I have less wetted surface area of hull in the water.
I therefore should go faster than other boat presuming same sail area , hull shape and skipper skills
Is this a fact or am I doing my maths wrong.

simon

 

[lpdsn]

I'm not sure of your thinking there. Your displacement determines, as the name implies, how much water you displace. Obviously different hull forms come into it but for boats of roughly the same form the one with a heavier displacement will displace more water and is going to have more surface area in contact with the water.

Wetted surface are only real has an effect on speed in very light winds. Once you go a bit faster wave making resistance predominates. And if you've ever raced OD in light airs you'll know that it is 90% skill of the crew.

 

[jdc]

If my boat has a heavier construction , as in the materials used are of a greater mass, than another boat of the same displacement, presumably I have less wetted surface area of hull in the water.
I therefore should go faster than other boat presuming same sail area , hull shape and skipper skills
Is this a fact or am I doing my maths wrong.

simon

If a boat displaces more (Archimedes' principle) than another with a similar hull shape it must have more wetted surface than a lighter boat, so more drag at low speeds. At moderate speeds the wave making drag dominates, and LWL is the most important factor so more LWL is good, despite weight. However at even higher speeds the ability of the boat to rise a bit out of the water, reducing wave drag, comes back into play making the force - speed graph for lighter boats much less steep, so light is again better.

But if your boat has the same displacement as another of identical shape their total mass must be identical and so it can't make any difference. Heavier construction (whatever that means) but the same mass must imply less mass somewhere else (eg water tanks or anchor chain), but that makes no odds to the drag.

 

[Wansworth]

If your boat is of the same displacement and it is of heavy construction weight must have been saved in other parts of the boat to have the same displacement.You will have the same wetted surface if a similar shape.

 

[neilf39]

If two boats have exactly the same hull shape and are the same size and same displacement then whether the weight is in construction or in the stores carried should make no difference to wetted area unless the weight is so badly distributed one boat is floating away from its design waterline. However it may impact on performance as where the weight is carried will make a difference to how the boat performs and behaves. A lightly constructed boat with all the weight in the keel will be stiffer. A heavily constructed boat with a lot of weight in the topsides will be more tender.

 

[lpdsn]

A heavily constructed boat with a lot of weight in the topsides will be more tender.

Albeit that is probably not a concern in light airs.

Standard racing practice is to send a couple of the hevies in the crew to the forecabin or to make them lie down on the foredeck. What you lose in trimming by the bow is more than made up for by the reduction in wetted surface area at the stern.

That said, in conditions where wetted surface area makes a difference most cruisers turn on the engine.

 

[johnalison]

I think there is some truth in gertha's reasoning. If wetted surface were unimportant at normal sailing speeds we wouldn't be worrying so much about keeping clean bottoms. However, waterline length is such an important factor in displacement yachts that for most of the time the larger yacht will win, and will probably cope with head seas better as well.

 

[geem]

I think there is some truth in gertha's reasoning. If wetted surface were unimportant at normal sailing speeds we wouldn't be worrying so much about keeping clean bottoms. However, waterline length is such an important factor in displacement yachts that for most of the time the larger yacht will win, and will probably cope with head seas better as well.

An interesting thread. I am not convinced waterline length is that important under normal sailing conditions. Wetted area seems to be a bigger hindrance to fast sailing in light airs. The trend for modern wide arsed boats means lots of wetted area that inhibits light wind performance unless you carry lots of sail area. Most of the standard production boats dont carry much sail area so in light winds they turn on the engine. It seems to me that the most efficient hull shape for maximum displacement and minimum wetted area is the profile of a barrel. Modern hull shapes are light on displacement and heavy on wetted area.

 

[Lon nan Gruagach]

An interesting thread. I am not convinced waterline length is that important under normal sailing conditions. Wetted area seems to be a bigger hindrance to fast sailing in light airs. The trend for modern wide arsed boats means lots of wetted area that inhibits light wind performance unless you carry lots of sail area. Most of the standard production boats dont carry much sail area so in light winds they turn on the engine. It seems to me that the most efficient hull shape for maximum displacement and minimum wetted area is the profile of a barrel. Modern hull shapes are light on displacement and heavy on wetted area.

Interesting that you consider "wide arsed boats" to have a greater wetted area....
Geometry basics, hydrodynamics aside for a moment. For a given displacement, the shape with the least wetted area would be a sphere... (volume to surface area ratio and all that m'larky)
Its easy to imagine why a pointy front will give a bit of go faster, LWL having its part too. But large LWL with pointy arse sort of avoids the help from the following wave... So, whdday end up with? a pointy front wide arsed boat...

 

[geem]

Interesting that you consider "wide arsed boats" to have a greater wetted area....
Geometry basics, hydrodynamics aside for a moment. For a given displacement, the shape with the least wetted area would be a sphere... (volume to surface area ratio and all that m'larky)
Its easy to imagine why a pointy front will give a bit of go faster, LWL having its part too. But large LWL with pointy arse sort of avoids the help from the following wave... So, whdday end up with? a pointy front wide arsed boat...

We are saying the same thing! A barrel is spherical in cross section.
Most boats are pointy at the front last time I looked but how you release the water off the stern has an impact to boat speed in light airs. Any dinghy racer knows to get the stern clear in light winds to go fast. This reduces the waterline length on most designs but you still go faster. Combine conservative sized rigs with wide arsed boats and in light airs you may be embarrassed what goes past you.

 

[johnalison]

We are saying the same thing! A barrel is spherical in cross section.
Most boats are pointy at the front last time I looked but how you release the water off the stern has an impact to boat speed in light airs. Any dinghy racer knows to get the stern clear in light winds to go fast. This reduces the waterline length on most designs but you still go faster. Combine conservative sized rigs with wide arsed boats and in light airs you may be embarrassed what goes past you.

I think that you have acquired one dimension too many; maybe you are living in a 4-dimensional world. A barrel may be circular in cross-section, as may a sphere, and also my boat for most of its length. In fact, my boat is a useful example, being an HR 34. This was replaced by HR with the 340, which matches it in most respects but is stretched to a greater waterline length. This allows it to be slightly faster off the wind, though its slightly greater sail area is part of the effect. I have though passed my mate with a 340 off the wind in light airs on one occasion, which seems to support the idea that wetted area matters. (This was early in the season and we both had clean bottoms).

 

[jdc]

Since I always like a graph, here's one for my boat (12.8m / 42'). It shows the speed at which the friction drag from the wetted area (green) and the wave-making drag (blue) are equal, and the steepness of the drag versus speed as it gets to 'hull speed'. A lighter boat would not have such a steep curve, but nonetheless it shows that friction is pretty significant and the two do not cross-over until going at over 6 knots.



I wrote up this modelling here.

Not my work, but I found this curve showing the effect of fouling in a splendid book on yacht design by Lars Larsson and Rolf Eliasson (not Elvestrom as I'd remembered incorrectly).



It shows that even 100µ (1/10th of a mm) of surface roughness makes a very significant difference to the friction, aka viscous, drag at any reasonable speed (10% at 4 knots) and a lot more if going faster. Worth keeping the hull clean!

 

[johnalison]

Interesting. I seem to remember reading that skin drag was important up to about 5kn, and since that was some years ago with slower boats that roughly matches. Your graph also makes me wonder just how much power my sails are giving on the rare occasions when we clock up 9-11 knots.

 

[jdc]

Interesting. I seem to remember reading that skin drag was important up to about 5kn, and since that was some years ago with slower boats that roughly matches. Your graph also makes me wonder just how much power my sails are giving on the rare occasions when we clock up 9-11 knots.

The hull shape and displacement are really important at higher speeds: my boat being heavy and conventionally shaped won't go above about 9 knots, but a more modern one would. See this from University of Delft:



The two hull shapes they studied show signifiant difference at high speeds.

 

[doug748]

Since I always like a graph, here's one for my boat (12.8m / 42'). It shows the speed at which the friction drag from the wetted area (green) and the wave-making drag (blue) are equal, and the steepness of the drag versus speed as it gets to 'hull speed'. A lighter boat would not have such a steep curve, but nonetheless it shows that friction is pretty significant and the two do not cross-over until going at over 6 knots.

View attachment 75586

I wrote up this modelling here.

.....

Great post.

The last graph in your paper: Equivalent wind force to a tidal flow , is going to raise a few eyebrows. Are you still happy with it?

 

[jdc]

Great post.

The last graph in your paper: Equivalent wind force to a tidal flow , is going to raise a few eyebrows. Are you still happy with it?

Thanks for your generous comment! Am I still happy with my last graph? Yes, at least with the numbers plotted but it's not that well explained perhaps: The wind force on my boat at a steady F4 is about 64 kg (I measured it) - not unreasonable I think. The speed through the water which generates this amount of drag is a bit over 5 knots, hence I say that a tide of a bit over 5 knots is equivalent to a F4 wind in terms of pull on the anchor. I admit I found this surprisingly low, and in contradiction to the received wisdom of avoiding strong tides when at anchor, but I still can't find anything wrong with the maths.

What it does not take into account, and what is much more important imho is the bad effect of a wind versus tide. My boat will always lie head to tide, and if it has a wind up its chuff it then shears about and tugs violently at its anchor at the limit of each shear. It is this which makes me dislike tides through my anchorage, not the force of the tide per-se. When wind and tide are from the same direction she lies quietly without noticeably greater tension in the anchor rode.

 

[Lon nan Gruagach]

Thanks for your generous comment! Am I still happy with my last graph? Yes, at least with the numbers plotted but it's not that well explained perhaps: The wind force on my boat at a steady F4 is about 64 kg (I measured it) - not unreasonable I think. The speed through the water which generates this amount of drag is a bit over 5 knots, hence I say that a tide of a bit over 5 knots is equivalent to a F4 wind in terms of pull on the anchor.

this is surprising to me too, even to the point of questioning a decimal point (no pun intended)..
I will only give a wee bit of thought to lifting 64kg.. however the prospect of holding onto a tow line thats pulling a 42' boat at 5 knots would certainly make me prefer a cleat!

 

[geem]

I think that you have acquired one dimension too many; maybe you are living in a 4-dimensional world. A barrel may be circular in cross-section, as may a sphere, and also my boat for most of its length. In fact, my boat is a useful example, being an HR 34. This was replaced by HR with the 340, which matches it in most respects but is stretched to a greater waterline length. This allows it to be slightly faster off the wind, though its slightly greater sail area is part of the effect. I have though passed my mate with a 340 off the wind in light airs on one occasion, which seems to support the idea that wetted area matters. (This was early in the season and we both had clean bottoms).

A few days ago we were sailing down the windward side of Grenada. Full sail on 12/14 kts on the beam. We were on a 44ft heavy ketch. We caught up a Ben 45. The current shaped one with the chines. He had a reef in the main. As soon as we got along side he shook out the reef. We both have slab reefing. We sailed along at similar speeds for a while. In the gusts we accelerated away a little to our surprise. We are an old fashion dipslacement hull that is hard to push above the therotical hull speed. We have minimal flat sections on the hull and we are heavy but well canvassed. I think from a quick Google search that the Ben 45 carries 1100sqft of sail. We carry 1250sqft. Having a spherical (barrel shaped) hull we have minimal wetted area for our displacement. In light winds where waves are not inducing planing we seem to have an advantage over a longer waterline length, larger boat with a wide stern. The Ben is considerably lighter and appeared to have a clean bottom. After 20nm we could hardly see the Ben and he eventually dropped sail as the wind went lighter. He resorted to engine, we carried on under goose wing as we rounded the island.

 

[jdc]

this is surprising to me too, even to the point of questioning a decimal point (no pun intended)..
I will only give a wee bit of thought to lifting 64kg.. however the prospect of holding onto a tow line thats pulling a 42' boat at 5 knots would certainly make me prefer a cleat!

I'm pretty confident the drag versus speed numbers are about right: the red lines are the output of my simplified, physics based, models but the red stars are the output of the well respected and semi-commercial hulldrag32 modelling program. The agreement is pretty good.